The use of fixtures for location and guidance of a drill bit to drill holes in a workpiece, such as a piece of wood, for pocket joints is well known. A pocket hole is one that is made at an angle in a working face of the wood workpiece and a screw is inserted into the hole to join the workpiece to another wood piece. The screw is recessed in the pocket hole and is not exposed.
U.S. Pat. No. 6,254,320, which is owned by the assignee of the subject application and the disclosure of which is incorporated herein by reference, discloses a relatively simple fixture for making pocket holes in a workpiece. In accordance with the invention of that patent, the fixture is U-shaped, with one leg being a guide leg against whose interior the broad working face of the workpiece in which a pocket hole to be drilled rests. The guide leg has angled channels to accept and guide a drill bit. There is a clamp leg opposite to the guide leg that has a threaded hole through which a screw clamp facing the guide leg passes and a base leg connects the guide and clamp legs to complete the U.
In the use of the fixture of U.S. Pat. No. 6,254,320, the narrow edge of the workpiece rests on the interior of the base leg and the broad working face in which the pocket hole is to be drilled is clamped against the interior surface of the guide leg by adjusting the threaded clamp to engage the opposite workpiece face. The distance between the base leg interior and drill bit guide channel exit sets the point on the workpiece at which the pocket hole is drilled. The clamp holds the workpiece securely against the guide leg while the drill bit is advanced through the angled guide channel to make an angled pocket hole in the workpiece. The fixture of the patent is simple in construction and efficient in operation permitting precise drilling of a pocket hole in a workpiece. However, it has a limitation in that its use is substantially limited to a workpiece of one thickness which is basically determined by the distance between the exit of the guide channel for the drill bit and the upper interior surface of the base leg on which the workpiece narrow edge rests.
U.S. Pat. No. 7,101,123, which also is owned by the assignee of this application and whose disclosure is incorporated by reference, discloses fixtures for drilling pocket holes in workpieces of different thicknesses by providing the fixture with the ability to vary the distance between the drill bit guide channel exit and the surface, or floor, on which the workpiece narrow edge rests. In one embodiment of this patent, a shelf is provided that is held between opposing sets of slots in or shoulders on the opposing faces of the guide leg and clamp leg. These slots or shoulders are spaced vertically apart and the workpiece edge rests on the shelf. The shelf is vertically adjustably positioned relative to the drill bit guide channel exit by selecting the set of slots in, or shoulders on, which it is placed. Each of the shelf vertical positions sets a different distance of the working face of the workpiece in which the pocket hole is to be drilled relative to the drill bit guide channel exit. This permits workpieces of different thicknesses to be accommodated for drilling of the pocket hole.
In another embodiment of U.S. Pat. No. 7,101,123, a shelf on which the workpiece edge rests has a neck that slides vertically in a cavity in the guide leg and the vertical position of the shelf is set by an adjustable screw that is threaded vertically through the top of the guide leg into the shelf neck. The vertical height of the shelf relative to the drill bit guide channel exit is set by rotating the screw. This sets the proper height for different thicknesses of workpieces that can be clamped against the guide leg for drilling of the pocket hole in the correct place on the workpiece working face. While this embodiment is fully workable, it has disadvantages in that it is somewhat costly to make. Also, the only point of stability for the shelf is the neck that is in the guide leg cavity and downward force on the shelf and workpiece can strip the vertical shelf positioning screw in the guide leg. Therefore, the shelf may not remain perfectly transverse to the guide wall at the desired height when loaded by the workpiece and added force is applied during drilling of a pocket hole.